In a printer, such as a laser printer, which prints an image on a sheet including various types of sheets such as cardboard, copying sheet and OHP films, information on a thickness and type of the sheet is required for optimizing various conditions for printing and fusing processes. Conventionally, such information specifying a type of sheet subjected to the print out is manually input by a user. However, such manual input deteriorates convenience for the user. Accordingly, a technique is desired, which automatically determines a type of a sheet with high accuracy before starting to feed and convey the sheet for the print out.
A conventional device for optically measuring a sheet thickness in a state where the sheets are stacked on a tray in the printer, which will be referred to as an optical sheet thickness measuring device, will be described. The optical sheet thickness measuring device measures the thickness of the sheet in a state where two or more sheets are stacked on top of one another before the sheets are conveyed. An example of such device is disclosed in JP-A-2005-104723.
As a main configuration, the conventional optical sheet thickness measuring device stores the sheets in a state where two or more sheets are stacked and aligned on a tray and illuminates a side face of the stack of sheets by a light source from obliquely above or from obliquely below in order to emphasize irregularity at the side face of the stack of sheets. A light reflected from an area where directly illuminated by the light source is captured by a light receiving element, and a peak-to-peak distance is calculated from a wave pattern of the received light regarding brightness and darkness that are caused by the stacked sheets, thereby obtaining the thickness of the sheet. Since a light intensity of the reflected light is large at edges of the sheets and the light intensity of the reflected light is small at gaps between the sheets, there appears a lower peak at the gaps in light amount.
However, when only the light reflected by the directly illuminated area of the side face of the sheet stack is captured, it is difficult to obtain from the peak such contrast that is sufficient to find the accurate thicknesses of the respective sheets. And there is a possibility that a thickness of two or more sheets instead of single sheet is detected, causing it difficult to detect the accurate thickness of the sheet. Also, since the thickness of each individual sheet ranges from 60 μm to 300 μm, the thickness of two or more sheets may be erroneously detected as the thickness of thick sheet if the sheet thickness is reliably detected.